PROJECT SUMMARY Sculpting tissues and organs into their 3D functional morphologies requires a tight spatiotemporal control of tissue mechanics. While cell-generated mechanical forces power morphogenesis, the resulting tissue flows that shape embryonic tissues in 3D depend strongly on the local tissue material properties, which govern the system's response to the internally generated forces. As a consequence, spatiotemporal variations in both mechanical forces and material properties can, independently or in combination, guide morphogenesis. The complexity of probing tissue mechanics within developing embryos has so far hindered our ability to dissect their specific roles and, more generally, to understand the biomechanical mechanisms that govern 3D tissue and organ morphogenesis. Using novel microdroplet-based techniques that the PI recently developed to measure both the tissue material properties and endogenous mechanical stresses within developing embryos, we propose to reveal the biomechanical mechanisms that underlie the formation of the zebrafish body axis. During posterior body axis elongation, cells display an anteroposterior gradient in their motility. Our preliminary data suggest that the anteroposterior variations in cellular movements may be caused by a transition between a fluid-like state of the tissue at the posterior end to a solid-like state in the presomitic mesoderm. Our hypothesis is that regional differences in fluid-like and solid-like tissue states control 3D tissue morphogenesis by enabling or restricting morphogenetic flows. Specifically, we hypothesize that during zebrafish body elongation the paraxial mesoderm transits from a fluid-like behavior in the tailbud to a solid-like behavior in the presomitic mesoderm, allowing tissue flows at the elongating body end while providing mechanical integrity to developmentally older structures, thereby guiding the nearly unidirectional tissue elongation of the body axis. In order to test this hypothesis, we plan to (1) measure and compare anteroposterior variations in tissue yield stress and endogenous mechanical stresses to establish the existence of fluid-like or solid-like tissue regions during body axis elongation, (2) establish how key functional molecules (actin, non-muscle myosin II and N-cadherin) control gradients in tissue mechanics and solid-like and fluid-like tissue states, and (3) integrate molecular, cell and tissue mechanics into a multiscale biomechanical model of body elongation. We believe this research will reveal a novel biomechanical mechanism of 3D tissue and organ morphogenesis, in which the spatial control of fluid-like and solid-like tissue regions guides the shaping of embryonic tissues. Moreover, it will dissect the specific roles of mechanical stresses and material properties in 3D tissue morphogenesis and establish how key functional molecules control tissue mechanics in vivo. !